1. Field of the Invention
The present invention relates to a four-cycle internal combustion engine which permits a substantial reduction of the harmful elements in the exhaust, particularly NOx and enhancement of fuel economy without detracting the engine performance even under an extraordinarily heavy EGR (exhaust gas recirculation).
2. Description of Prior Art
Lately with introduction of strengthened measures for purification of auto exhaust gases a substantial reduction of the volume of NOx to be released into the atmosphere from automobiles under light and medium loads is being demanded.
Reduction of NOx can be carried out by two methods: The so-called pre-treatment, in which the gas mixture is burned such that the proportion of NOx released into the atmosphere can be suppressed lower; and the so-called post-treatment, in which NOx in the exhaust gases is rendered harmless with use of a reducing catalyst, etc.
It is common practice in the post-treatment to adopt a ternary catalyst (which possesses both the oxidizing and the reducing functions) for the purpose of reducing HC and CO too. In this case, however, the cost becomes very high, since an O.sub.2 -sensor for detecting the oxygen concentration in the exhaust gases and an electronically controlled mixed-gas generator for obtaining a highly precise air-fuel ratio are required.
In the case of the pre-treatment, it is common practice to make EGR for the purpose of reducing NOx and at the same time generate a swirl of the mixed gas in the combustion chamber, thereby increasing the combustion spread speed so that the engine performance may not deteriorate under EGR.
In this method, however, the swirl generated is not strong enough and in consequence the rate of EGR (the weight of exhaust gas against the total weight of suction gas) remains 10% at the most. Thus it has been impossible by EGR alone to reduce the volume of NOx to the level specified in the latest requirement. For this reason, in the current practice, besides EGR the spark is advanced before top dead center and a mixed gas of higher concentration than the theoretical air-fuel ratio is supplied, resulting in a poor fuel economy.